Lost circulation composition for fracture sealing

ABSTRACT

A lost circulation material and method for well treatment employing the material that is effective at sealing or plugging fractured zones and has utility over a wide range of temperatures, including high temperatures. The lost circulation material includes particulate material to quickly de-fluidize the fluid formulation, fibrous material to suspend particles in the slurrified form of the composition and increase the shear strength of the resultant seal, and non-Portland cement material for increasing the compressive strength.

FIELD OF THE INVENTION

The present embodiments generally relate to subterranean cementing operations and, more particularly, to methods and compositions for preventing or alleviating the loss of drilling fluids and other well servicing fluids into subterranean formations during drilling or construction of boreholes in such subterranean formations.

BACKGROUND

The following paragraphs contain some discussion, which is illuminated by the innovations disclosed in this application, and any discussion of actual or proposed or possible approaches in this Background section does not imply that those approaches are prior art.

Natural resources such as oil and gas residing in a subterranean formation or zone are usually recovered by forming a wellbore that extends into the formation. The wellbore is drilled while circulating a drilling fluid therein. The drilling fluid is usually circulated downwardly through the interior of a drill pipe and upwardly through the annulus, which is located between the exterior of the pipe and the walls of the wellbore. After terminating the circulation of the drilling fluid, a string of pipe, e.g., casing, is run in the wellbore. Next, primary cementing is typically performed by pumping cement slurry into the annulus and allowing the cement to set into a hard mass (i.e., sheath). The cement sheath attaches the string of pipe to the walls of the wellbore and seals the annulus.

Often in drilling a wellbore, one or more pervious zones are encountered. The pervious zones may be, for example, highly permeable, unconsolidated, vugs, voids, naturally occurring fractures, or induced fractures that occur when weak zones have fracture gradients exceeded by the hydrostatic pressure of the drilling fluid or the cement slurry. During the drilling operation, the pervious or thief zones may result in the loss of drilling fluid. The drilling fluid flows into the thief zones rather than being returned to the surface, which reduces circulation of the drilling fluid. When circulation is lost, pressure on the open formation is reduced, which can result in an undesired zone flowing into the well or even catastrophic loss of well control.

A large variety of materials have been used or proposed in attempts to cure lost circulation. Generally, such materials are divided into four types or categories: fibrous materials, such as monofilament synthetic fibers; flaky materials, such as wood chips or mica flakes; granular materials, such as ground marble or petroleum coke; and settable compositions, the relative strength of which increases upon a preplanned mode of triggering after placement, such as hydraulic cement.

Although many materials and compositions exist and have been proposed for preventing lost circulation, there continues to be a need for even more versatile and better compositions and methods for preventing, as well as mitigating, loss of circulation.

SUMMARY OF THE INVENTION

According to several exemplary embodiments, an improved lost circulation material (LCM) includes a combination of several materials to obtain a composition that enables the placement of a firm, immobile mass in a fracture, to prevent lost circulation of drilling fluid, which cannot be pressed out of the fracture in part or whole, by pressure fluctuations.

According to several exemplary embodiments, the improved lost circulation material for plugging fractures includes particulate material to quickly de-fluidize the fluid formulation, fibrous material to suspend particles in the slurrified form of the composition and increase the shear strength of the resultant seal and non-Portland cement material for increasing the compressive strength.

According to several exemplary embodiments, the lost circulation material composition is used to prevent or alleviate loss of drilling fluid or other fluid circulation in a wellbore penetrating a subterranean formation. According to several exemplary embodiments, the lost circulation material composition is provided in a weighted or unweighted “pill” for introduction into the wellbore. Such “pills” typically comprise the lost circulation material composition blended with a required amount of water, base oil, water based drilling fluid or non-aqueous based drilling fluid and, in some cases, a weighting agent such as barite, salt or calcium carbonate. The amount of the lost circulation material composition used in the pill will depend on the size of the subterranean fracture, opening or lost circulation zone to be treated. Multiple pills or treatments may be used if needed. According to several exemplary embodiments, drilling is stopped while the pill comprising the lost circulation material composition of the present invention is introduced into the wellbore. The lost circulation material composition will enter lost circulation zones or porous or fractured portions of the formation where it will prevent or retard the entry of drilling and other wellbore fluids. Pressure can be used to squeeze the pill into the lost circulation zone and de-fluidize the slurry.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE depicts a schematic view of a wellbore drilling assembly, according to several exemplary embodiments.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides many different embodiments, or examples, of the present invention for implementing different features of various embodiments of the present invention. Specific examples of components are described below to simplify and exemplify the present disclosure. These are, of course, merely exemplary and are not intended to be limiting.

According to several exemplary embodiments, an improved lost circulation material (LCM) includes a combination of several materials to obtain a composition that enables the placement of a firm, immobile mass in a fracture, to prevent lost circulation of drilling fluid, in which the composition cannot be pressed out of the fracture in whole or in part by pressure fluctuations.

The lost circulation material composition is a high-fluid-loss-squeeze material which gains both compressive strength and shear strength when the material is de-fluidized in a fractured zone. For example, the lost circulation material may be used to seal fractured shale formations.

According to several exemplary embodiments, the lost circulation material efficiently seals pores and stops drilling fluid losses through large fractures, such as those having a size of about 200 microns or more, and in some embodiments fractures having a size of about 200 microns to about 4000 microns, and in other embodiments fractures having a size of about 500 microns to about 2500 microns, while showing tolerance to high temperatures such as in some embodiments from about 70° F. to about 400° F. and in other embodiments from about 150° F. to about 250° F.

According to several exemplary embodiments, the lost circulation material composition for plugging fractures includes particulate material to quickly de-fluidize the fluid formulation, fibrous material to suspend particles in the slurrified form of the composition and increase the shear strength of the resultant seal and non-Portland cement material for increasing the compressive strength.

According to several exemplary embodiments, the particulate material of the lost circulation material composition for plugging fractures includes one or more of diatomaceous earth, graphitic material and ground cellulosic material. According to several exemplary embodiments, the ground cellulosic material includes one or more of ground nut shells and ground fibrous cellulosic material.

According to several exemplary embodiments, the ground nut shells include one or more of walnut shells, peanut shells, almond shells, cashew shells, brazil nut shells, chestnut shells, pistachio shells and pecan shells. According to several exemplary embodiments, the ground nut shells include one or both of walnut shells and pecan shells.

According to several exemplary embodiments, the fibrous material includes synthetic chopped fibers, such as one or more of polyacrylonitrile fibers, acrylonitrile/methyl acrylate copolymer fibers, polypropylene fibers, viscose fibers, carbon fibers, silicon carbide fibers, fiberglass fibers, acrylic polyester fibers, polyamide fibers, aromatic polyamide fibers, polyolefin fibers, polyurethane fibers, polyvinyl chloride fibers and polyvinyl alcohol fibers, having an average length of about 0.5 to 13 millimeters. In several exemplary embodiments, the synthetic chopped fibers have an average length of about 1 to 6 millimeters. In still other embodiments, the synthetic chopped fibers have an average length of about 3 millimeters or about 6 millimeters.

According to several exemplary embodiments, the non-Portland cement material of the lost circulation material composition for plugging fractures includes a cement set accelerator. According to several exemplary embodiments, the cement set accelerator includes one or more of calcium sulfate hemihydrate and sodium metasilicate. In several exemplary embodiments, the sodium metasilicate is in anhydrous form.

According to several exemplary embodiments, the lost circulation material for plugging fractures includes the following components: about 45-60% by weight of diatomaceous earth, about 5-15% by weight of graphitic material, such as STEELSEAL® graphitic material which is commercially available from Halliburton Energy Services, Inc., about 5-15% by weight of ground nut shells, about 5-15% by weight of ground fibrous cellulosic material, about 5-15% by weight of synthetic chopped fibers, about 10-20% by weight of calcium sulfate hemihydrate and about 2-12% by weight of sodium metasilicate.

The diatomaceous earth, the graphitic carbon, the ground nut shells, and the ground fibrous cellulosic material are particulate lost circulation materials and allow quick de-fluidization of the formulations. The synthetic chopped fibers are lost circulation materials that seal the thief zone, that suspend particles in the slurrified form of the formulations and increase the shear strength of the resultant seal. The cement set accelerators are non-Portland cement materials for increasing the compressive strength of the formulations.

According to several exemplary embodiments, the lost circulation material for plugging fractures forms an unweighted filter cake having a relative shear strength of up to about 1325 psi at room temperature. For the purpose of this patent application “relative shear strength” shall be referred to herein simply as “shear strength”. According to several exemplary embodiments, the lost circulation material for plugging fractures forms an unweighted filter cake having a shear strength per dtex of the synthetic chopped fibers of up to about 1366 psi/dtex at room temperature. The term “dtex” is a unit of measure for the linear mass density of fibers and is defined as the mass in grams per 10,000 meters.

According to several exemplary embodiments, the lost circulation material for plugging fractures forms a 12.0 lb/gal filter cake having a shear strength of up to about 420 psi at room temperature. According to several exemplary embodiments, the lost circulation material for plugging fractures forms 12.0 lb/gal filter cake having a shear strength per dtex of the synthetic chopped fibers of up to about 432 psi/dtex at room temperature.

The carrier fluid for the lost circulation material may be water, base oil, water based drilling fluid, and non-aqueous based drilling fluid. According to several exemplary embodiments, the lost circulation material is added to the carrier fluid in an amount of up to about 70 pounds per barrel (ppb). Also, according to several exemplary embodiments, a weighted lost circulation material composition can be prepared by adding barite, salt, calcium carbonate or other conventional weighting materials to the fluid to achieve a desired density. A suitable barite weighting material is Baroid® 41 weighting material which is commercially available from Halliburton Energy Services, Inc.

In operation, the lost circulation material is mixed with the carrier fluid to form a lost circulation pill and pumped into a wellbore penetrating a subterranean zone. Once the pill has been spotted into the thief zone, squeeze pressure from the surface causes the lost circulation pill to lose fluid quickly to the permeable formation or to the pervious fracture network. The immobile mass that forms gains both compressive and shear strength while in place in a fractured or other pervious zone and plugs the fractured or other pervious zone. The sealing mass quickly sets into a rigid sealing mass that is substantially impermeable to whole drilling fluid such that minimal subsequent drilling or treatment fluids pass into the fractured or other pervious zone.

The exemplary lost circulation material disclosed herein may directly or indirectly affect one or more components or pieces of equipment associated with the preparation, delivery, recapture, recycling, reuse and/or disposal of the disclosed lost circulation material. For example, and with reference to the FIGURE, the disclosed lost circulation material may directly or indirectly affect one or more components or pieces of equipment associated with an exemplary wellbore drilling assembly 100, according to one or more embodiments. It should be noted that while the FIGURE generally depicts a land-based drilling assembly, those skilled in the art will readily recognize that the principles described herein are equally applicable to subsea drilling operations that employ floating or sea-based platforms and rigs, without departing from the scope of the disclosure.

As illustrated, the drilling assembly 100 may include a drilling platform 102 that supports a derrick 104 having a traveling block 106 for raising and lowering a drill string 108. The drill string 108 may include, but is not limited to, drill pipe and coiled tubing, as generally known to those skilled in the art. A kelly 110 supports the drill string 108 as it is lowered through a rotary table 112. A drill bit 114 is attached to the distal end of the drill string 108 and is driven either by a downhole motor and/or via rotation of the drill string 108 from the well surface. As the bit 114 rotates, it creates a borehole 116 that penetrates various subterranean formations 118.

A pump 120 (e.g., a mud pump) circulates drilling fluid 122, through a feed pipe 124 and to the kelly 110, which conveys the drilling fluid 122 downhole through the interior of the drill string 108 and through one or more orifices in the drill bit 114. The drilling fluid 122 is then circulated back to the surface via an annulus 126 defined between the drill string 108 and the walls of the borehole 116. At the surface, the recirculated or spent drilling fluid 122 exits the annulus 126 and may be conveyed to one or more fluid processing unit(s) 128 via an interconnecting flow line 130. After passing through the fluid processing unit(s) 128, a “cleaned” drilling fluid 122 is deposited into a nearby retention pit 132 (i.e., a mud pit). While illustrated as being arranged at the outlet of the wellbore 116 via the annulus 126, those skilled in the art will readily appreciate that the fluid processing unit(s) 128 may be arranged at any other location in the drilling assembly 100 to facilitate its proper function, without departing from the scope of the disclosure.

One or more components of the disclosed lost circulation material may be added to the drilling fluid 122 via a mixing hopper 134 communicably coupled to or otherwise in fluid communication with the retention pit 132. The mixing hopper 134 may include, but is not limited to, mixers and related mixing equipment known to those skilled in the art. In other embodiments, however, the disclosed components of the lost circulation material may be added to the drilling fluid 122 at any other location in the drilling assembly 100. In at least one embodiment, for example, there could be more than one retention pit 132, such as multiple retention pits 132 in series. Moreover, the retention put 132 may be representative of one or more fluid storage facilities and/or units where the disclosed components of the lost circulation material may be stored, reconditioned and/or regulated until added to the drilling fluid 122.

As mentioned above, the disclosed lost circulation material may directly or indirectly affect the components and equipment of the drilling assembly 100. For example, the disclosed lost circulation material may directly or indirectly affect the fluid processing unit(s) 128 which may include, but is not limited to, one or more of a shaker (e.g., shale shaker), a centrifuge, a hydrocyclone, a separator (including magnetic and electrical separators), a desilter, a desander, a separator, a filter (e.g., diatomaceous earth filters), a heat exchanger and any fluid reclamation equipment. The fluid processing unit(s) 128 may further include one or more sensors, gauges, pumps, compressors, and the like used store, monitor, regulate and/or recondition the exemplary lost circulation material.

The disclosed lost circulation material may directly or indirectly affect the pump 120, which representatively includes any conduits, pipelines, trucks, tubulars and/or pipes used to fluidically convey the lost circulation material downhole, any pumps, compressors, or motors (e.g., topside or downhole) used to drive the lost circulation material into motion, any valves or related joints used to regulate the pressure or flow rate of the lost circulation material and any sensors (i.e., pressure, temperature, flow rate, etc.), gauges, and/or combinations thereof and the like. The disclosed lost circulation material may also directly or indirectly affect the mixing hopper 134 and the retention pit 132 and their assorted variations.

The disclosed lost circulation material may also directly or indirectly affect the various downhole equipment and tools that may come into contact with the lost circulation material such as, but not limited to, the drill string 108, any floats, drill collars, mud motors, downhole motors and/or pumps associated with the drill string 108 and any MWD/LWD tools and related telemetry equipment, sensors or distributed sensors associated with the drill string 108. The disclosed lost circulation material may also directly or indirectly affect any downhole heat exchangers, valves and corresponding actuation devices, tool seals, packers and other wellbore isolation devices or components and the like associated with the wellbore 116. The disclosed lost circulation material may also directly or indirectly affect the drill bit 114, which may include, but is not limited to, roller cone bits, PDC bits, natural diamond bits, any hole openers, reamers, coring bits, etc.

While not specifically illustrated herein, the disclosed lost circulation material may also directly or indirectly affect any transport or delivery equipment used to convey the lost circulation material to the drilling assembly 100 such as, for example, any transport vessels, conduits, pipelines, trucks, tubulars and/or pipes used to fluidically move the lost circulation material from one location to another, any pumps, compressors, or motors used to drive the lost circulation material into motion, any valves or related joints used to regulate the pressure or flow rate of the lost circulation material and any sensors (i.e., pressure and temperature), gauges, and/or combinations thereof and the like.

The following examples are illustrative of the compositions and methods discussed above.

EXAMPLES

Certain embodiments of lost circulation material formulations according to the present invention are shown below in Table 1 and were readily mixed into water or ENCORE® BASE, an olefin-based synthetic base oil which is commercially available from Halliburton Energy Services, Inc. The lost circulation material formulations were prepared by simple mixing of ingredients and are set forth as a percentage by weight of the dry composition.

TABLE 1 High Fluid-Loss Squeeze Formulations Material (%) H-SA-1 H-SA-2 H-SA-3 H-SA-4 STEELSEAL ® 1000 5 5 5 graphitic material STEELSEAL ® 400 graphitic 5 material Walnut F 10 Pecan M 10 10 10 BAROFIBRE ® O finely 5 5 5 5 ground fibrous cellulosic material Diatomaceous Earth Mn-84 54 50 52 50 3 mm polypropylene fiber 6 10 8 6 CAL-SEAL 60 ™ calcium 14 14 14 18 sulfate hemihydrate cement set accelerator ECONOLITE ™ sodium 6 6 6 6 metasilicate cement set accelerator

As noted above, STEELSEAL® graphitic material is commercially available from Halliburton Energy Services, Inc. STEELSEAL® 1000 graphitic material and STEELSEAL® 400 graphitic material are resilient, angular, dual-composition carbon-based lost circulation materials having particle sizes of 1000±100 microns and 400±50 microns, respectively. Walnut F and Pecan M are ground walnut and pecan shells, respectively, which are commercially available from Grinding & Sizing Co., Inc. BAROFIBRE® O finely ground fibrous cellulosic material is commercially available from Halliburton Energy Services, Inc. Diatomaceous Earth Mn-84 is commercially available from EP Minerals. Cal-Seal 60™ calcium sulfate hemihydrate cement set accelerator is commercially available from Halliburton Energy Services, Inc. Econolite™ sodium metasilicate cement set accelerator is commercially available from Halliburton Energy Services, Inc.

As shown in Table 2 below, filter cakes of the lost circulation material formulations set forth in Table 1 above, as well as EZ SQUEEZE® lost circulation material and WEDGE-SET® lost circulation material, which are commercially available from Turbo-Chem International Inc. and Sharp-Rock Technologies, Inc., respectively, were prepared under various conditions and then tested in terms of shear strength.

TABLE 2 Comparative Shear Strength Analysis EZ- SQUEEZE ® WEDGE- lost SET ® lost circulation circulation H-SA-1 H-SA-2 H-SA-3 H-SA-4 material material Un-weighted, dewatered 1197 psi 1810 psi 1420 psi 780 psi 967 psi 290 psi at room temp. Un-weighted, dewatered 1322 psi 2458 psi 1898 psi 1035 psi  263 psi at 200° F. 12.5 lb/gal, dewatered at  590 psi  760 psi  740 psi 530 psi  80 psi 200° F. Un-weighted, ENCORE 1210 psi 200 psi at room temp. 12.5 lb/gal, ENCORE at  230 psi  80 psi room temp. Un-weighted, ENCORE 1650 psi 490 psi at 200° F. 12.5 lb/gal ENCORE at  260 psi 160 psi 200° F.

Shear strength was measured using a push-out apparatus. The push-out apparatus measures shear strength by applying pressure onto a small portion of a confined composite. Once the composite is formed, it is placed into a close-fitting sample holder, which has a hole at the bottom that is approximately one-half the diameter of the composite. Force is applied via a push-out piece or piston using a Carver Press. The sample holder is raised up, allowing some portion of the composite to be free to come out through the hole. Pressure is linearly applied over time until failure of the composite has been reached which is the yield point. The maximum force in pounds from the gauge is recorded. The recorded pressure is converted to shear strength as follows:

Failure area (A)=π*d*t

S=(F)/A

-   -   Where d is the plug diameter (in), t is the composite thickness         (in), A is the failure area (in²), F is the maximum recorded         force (lbf), and S is the shear strength (psi).

The height of the filter cake for each sample and the pounds of force required to push-out a sample of the cake were measured and then used to calculate the shear strength. The height of the cake was typically between 10 and 15 mm before the test.

When filter cakes of the lost circulation material compositions of the present invention were made at room temperature, the shear strengths varied from 780 to 1810 psi as shown in TABLE 2. These results compare favorably to the results obtained for EZ-SQUEEZE® lost circulation material and WEDGE-SET® lost circulation material and, specifically, the shear strength of the H-SA-2 formulation is 1.87 and 6.25 times their shear strength, respectively.

The shear strength of the H-SA-1, H-SA-2 and H-SA-3 formulations was also determined for filter cakes made at 200° F. and the results are also shown in TABLE 2. Again, the results compare favorably to those obtained for EZ-SQUEEZE® lost circulation material and WEDGE-SET® lost circulation material and, specifically, the shear strength of the H-SA-2 formulation is 2.35 and 9.35 times their shear strength, respectively.

The shear strength of the H-SA-1, H-SA-2 and H-SA-3 formulations at 12.5 lb/gal are also shown in TABLE 2 and again the shear strength of the H-SA-2 formulation is 1.4 times higher compared to EZ-SQUEEZE® lost circulation material while the shear strength for WEDGE-SET® lost circulation material is less than 100 psi.

The shear strengths for H-SA-2 and EZ-SQUEEZE® were determined for un-weighted and 12.5 lb/gal fluid formulations using ENCORE® BASE olefin-based synthetic base oil which is commercially available from Halliburton Energy Services, Inc., as the carrier fluid, rather than water. Using ENCORE® BASE olefin-based synthetic base oil as the carrier fluid, un-weighted filter cakes made at room temperature with H-SA-2 have a shear strength that is about 6 times higher than those made with EZ-SQUEEZE® lost circulation material. Again using ENCORE® BASE olefin-based synthetic base oil as the carrier fluid, the 12.5 lb/gal filter cakes made at room temperature with H-SA-2 have a shear strength that is about 2.9 times higher than those made with EZ-SQUEEZE® lost circulation material as shown in TABLE 2. At 200° F. and using ENCORE® BASE olefin-based synthetic base oil as the carrier fluid, the shear strength for H-SA-2 is about 3.33 times higher in the un-weighted fluid and about 1.65 times higher in the 12.5 lb/gal fluid than EZ-SQUEEZE® lost circulation material as shown in TABLE 2.

As shown in TABLE 3 below, filter cakes of certain embodiments of lost circulation material formulations according to the present invention as well as EZ SQUEEZE® lost circulation material and WEDGE-SET® lost circulation material which are commercially available from Turbo-Chem International Inc. and Sharp-Rock Technologies, Inc., respectively, were prepared under various conditions and then tested in terms of unconfined compressive strength.

TABLE 3 Comparative Unconfined Compressive Strength EZ-SQUEEZE ® WEDGE-SET ® lost circulation lost circulation H-SA-1 H-SA-2 H-SA-3 material material Un-weighted, >950 psi >950 psi  >950 psi >950 psi 600 psi dewatered at 200° F. 12.5 lb/gal, dewatered  160 psi 910 psi  340 psi 230 psi 170 psi at 200° F. Un-weighted, >1080 psi  >1080 psi ENCORE ® at 200° F. 12.5 lb/gal, ENCORE ® 520 psi 75 psi at 200° F.

Unconfined compressive strength was measured using a Carver Press where the height of the filter cake was equal to or greater than the diameter of the filter cake. The filter cakes were formed at room temperature and 200° F. using 40 micrometer aloxite (aluminum oxide) disks.

The unconfined compressive strengths for the un-weighted and 12.5 lb/gal fluid formulations containing H-SA-1, H-SA-2 and H-SA-3 were determined at 200° F. The unconfined compressive strengths for the un-weighted formulations were all at least 950 psi for the experimental formulations and EZ-SQUEEZE® lost circulation material, but only 600 psi for WEDGE-SET® lost circulation material. The unconfined compressive strengths for the filter cakes made from 12.5 lb/gal formulations are shown in TABLE 3 and the unconfined compressive strength when using H-SA-2 is about 4 times higher than that for EZ-SQUEEZE® lost circulation material.

The unconfined compressive strength for un-weighted filter cakes of H-SA-2 and EZ-SQUEEZE® lost circulation material using ENCORE® BASE olefin-based synthetic base oil as the carrier fluid, are at least 1080 psi when made at 200° F., but the unconfined compressive strengths drop to 520 psi and 75 psi, respectively when at 12.5 lb/gal. Nevertheless, the unconfined compressive strength at 200° F. when using ENCORE® BASE olefin-based synthetic base oil and H-SA-2 is about 6.9 times higher than when using ENCORE® BASE olefin-based synthetic base oil and EZ-SQUEEZE® lost circulation material.

Slot plugging tests were conducted using 1, 1.5, 2 and 2.5 mm slotted disks as well as a tapered slot (2.5 to 1 mm over 37 mm) to determine if the un-weighted formulation is useful in a range of fracture sizes. All of the un-weighted formulations bridged the slotted disks at a 50 lb/bbl concentration which demonstrates that the formulations are useful in a wide range of lost circulation situations.

Pumpability was measured using a non-positive displacement pump head attachment in place of the emulsion screen on a Silverson Mixer at 3000 rpm for 5 minutes. Two lab barrel quantities of H-SA-1 and H-SA-2 were mixed and then pumped through a 6 mm tube using the Silverson Mixer equipped with a non-positive displacement pump head. Neither of the formulations plugged the tube during the test. When a two barrel mixture of 50 lb/bbl EZ-SQUEEZE® lost circulation material was pumped through the same setup, it plugged the tube within 15 seconds of starting the test.

Fluid loss was measured using a standard API fluid loss apparatus with 100 psi differential at room temperature. Un-weighted fluids containing H-SA-1 or H-SA-2 were dewatered in the API filter press giving >50 ml filtrate over about 15 seconds.

Shown below in TABLE 4 is the formulation for a 12.5 lb/gal BORE-MAX® drilling fluid which was used to determine if the H-SA-2 formulation would still lose fluid quickly after being contaminated with a drilling fluid.

TABLE 4 Formulation for 12.5 lb/gal BORE-MAX ® Drilling Fluid Material Amount (g) Water 308.43 BORE-VIS ® II viscosifier 4 BORE-PLUS ™ suspension agent 2 POLYAC ® PLUS filtration control agent 1 Barite 197 BARAZAN ® D PLUS viscosifier 0.25 BARABUF ® pH buffer 0.25

In terms of the components of BORE-MAX® drilling fluid, BORE-VIS® II is a viscosifier, BORE-PLUS™ is a suspension agent, POLYAC® PLUS is a filtration control agent, BARAZAN® D PLUS is a viscosifier and BARABUF® is a pH buffer, all of which are commercially available from Halliburton Energy Services, Inc. Barite is barium sulfate and is widely commercially available.

When an un-weighted fluid formulation of H-SA-2 was contaminated by a 10% addition of 12.5 lb/gal BORE-MAX® drilling fluid, the filtration rate decreased from >50 ml in 30 seconds to 31 ml in 30 seconds. Reducing the amount of contamination to 5% resulted in a filtration rate of 42 ml in 30 seconds. When the un-weighted fluid was contaminated with 4% BORE-MAX® drilling fluid, the filtration rate was 50 ml in 22 seconds. The 12.5 lb/gal fluid formulation containing H-SA-2 tolerated the 10% addition of BORE-MAX® drilling fluid and had a filtration rate of 50 ml in 20 seconds. A filter cake containing un-weighted H-SA-2 and 10% BORE-MAX® drilling fluid was measured to have a shear strength of 1850 psi, after it was dewatered at 200° F. A filter-cake containing 12.5 lb/gal H-SA-2 and 10% BORE-MAX® drilling fluid was measured to have a shear strength of 740 psi, after it was dewatered at 200° F.

Certain embodiments of lost circulation material formulations are shown below in Table 5 and were readily mixed into water. The lost circulation material formulations were prepared by simple mixing of ingredients and are set forth as a percentage by weight of the dry composition.

TABLE 5 High Fluid-Loss Squeeze Formulations Material (%) H-SA-5 H-SA-6 H-SA-7 H-SA-8 STEELSEAL ® 1000 5 5 5.26 5.26 graphitic material Pecan M 10 10 10.53 10.53 BAROFIBRE ® O finely 5 5 5.26 5.26 ground fibrous cellulosic material Diatomaceous Earth Mn-84 50 50 52.63 52.63 ⅛ inch 3.3 dtex 10 polypropylene fibers 3 mm 0.97 dtex 10 acrylonitrile/methyl acrylate fibers 6 mm 3.3 dtex polypropylene 5.26 fibers 6 mm 0.97 dtex 5.26 acrylonitrile/methyl acrylate fibers CAL-SEAL 60 ™ calcium 14 14 14.74 14.74 sulfate hemihydrate cement set accelerator ECONOLITE ™ sodium 6 6 6.32 6.32 metasilicate cement set accelerator

The polypropylene fibers are commercially available from Forta Corporation under the trade name FORTA-FERRO®. The acrylonitrile/methyl acrylate fibers are commercially available from Fabpro Polymers under the trade name AcryLok™.

As shown in Table 6 below, filter cakes of the lost circulation material formulations set forth in Table 5 above were prepared under various conditions and then tested in terms of shear strength at room temperature.

TABLE 6 Comparative Shear Strength Analysis H-SA-5 H-SA-6 H-SA-7 H-SA-8 Un-weighted 2323 psi 1325 psi 991 psi 734 psi 704 psi/dtex 1366 psi/dtex 300 psi/dtex 756 psi/dtex 2130 psi 1225 psi 645 psi/dtex 1263 psi/dtex 12.0 lb/gal 550 psi 420 psi 167 psi/dtex 432 psi/dtex

Shear strength was measured using a push-out apparatus as described above. Also as described above, the height of the filter cakes for each sample and the pounds of force required to push-out a sample of the cake were measured and then used to calculate the shear strength. The height of the cake was typically between 10 and 15 mm before the test.

As shown in TABLE 6, when filter cakes of the lost circulation material compositions were made at room temperature, the compositions that included the acrylonitrile methyl acrylate fibers had a higher shear strength per dtex (psi/dtex) value than similar compositions that included polypropylene fibers. The psi/dtex value was calculated in order to normalize the shear strength of the filter cakes resulting from the dewatering of the compositions shown in TABLE 5. Specifically, the unweighted lost circulation material compositions that included 3 millimeter (mm), 0.97 dtex acrylonitrile/methyl acrylate fibers had a shear strength of 1325 psi and 1225 psi, respectively, and a shear strength per unit thickness of the fibers of 1366 psi/dtex and 1263 psi/dtex, respectively. These results compare favorably to the results obtained for the unweighted lost circulation material compositions that included ⅛ inch, 3.3 dtex polypropylene fibers which had a shear strength of 2323 psi and 2130 psi, respectively, and a shear strength per unit thickness of the fibers of 704 psi/dtex and 645 psi/dtex, respectively.

In addition, as shown in TABLE 6, the unweighted and weighted (12 ppg) lost circulation material compositions that included 6 mm, 0.97 dtex acrylonitrile/methyl acrylate fibers had a shear strength of 734 psi and 420 psi, respectively, and a shear strength per unit thickness of the fibers of 756 psi/dtex and 432 psi/dtex, respectively. These results compare favorably to the results obtained for the unweighted and weighted (12 ppg) lost circulation material compositions that included 6 mm, 3.3 dtex polypropylene fibers which had a shear strength of 991 psi and 550 psi, respectively, and a shear strength per unit thickness of the fibers of 300 psi/dtex and 167 psi/dtex, respectively.

In summary, the results shown in TABLE 6 demonstrate that if the filter cakes that include the acrylonitrile/methyl acrylate fibers had been made with fibers having the same thickness as the polypropylene fibers, the filter cakes that included the acrylonitrile/methyl acrylate fibers would have a higher shear strength.

While the present invention has been described in terms of several exemplary embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.

The present disclosure has been described relative to several exemplary embodiments. Improvements or modifications that become apparent to persons of ordinary skill in the art only after reading this disclosure are deemed within the spirit and scope of the application. It is understood that several modifications, changes and substitutions are intended in the foregoing disclosure and in some instances some features of the invention will be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention. 

What is claimed is:
 1. A lost circulation material composition, comprising: particulate material; fibrous material comprising synthetic chopped fibers selected from the group consisting of polyacrylonitrile fibers and acrylonitrile/methyl acrylate fibers; and non-Portland cement material.
 2. The lost circulation material composition of claim 1, wherein the particulate material is selected from the group consisting of diatomaceous earth, graphitic material and ground cellulosic material.
 3. The lost circulation material composition of claim 2, wherein the ground cellulosic material is selected from the group consisting of ground nut shells and ground fibrous cellulosic material.
 4. The lost circulation material composition of claim 3, wherein the ground nut shells are selected from the group consisting of walnut shells, peanut shells, almond shells, cashew shells, brazil nut shells, chestnut shells, pistachio shells and pecan shells.
 5. The lost circulation material composition of claim 3, wherein the ground nut shells are selected from the group consisting of walnut shells and pecan shells.
 6. The lost circulation material composition of claim 1, wherein the synthetic chopped fibers have an average length of from about 0.5 to 13 millimeters.
 7. The lost circulation material composition of claim 6, wherein the synthetic chopped fibers have an average length of from about 1 to 6 millimeters.
 8. The lost circulation material composition of claim 6, wherein the synthetic chopped fibers have an average length of about 3 millimeters.
 9. The lost circulation material composition of claim 6, wherein the synthetic chopped fibers have an average length of about 6 millimeters.
 10. The lost circulation material composition of claim 6, wherein the synthetic chopped fibers have a dtex of 3.3.
 11. The lost circulation material composition of claim 1, wherein the non-Portland cement material comprises a cement set accelerator.
 12. The lost circulation material composition of claim 11, wherein the cement set accelerator is selected from the group consisting of calcium sulfate hemihydrate and sodium metasilicate.
 13. The lost circulation material composition of claim 1, comprising: 45 to 60 weight percent of diatomaceous earth; 5 to 15 weight percent of graphitic material; 5 to 15 weight percent of ground nut shells; 5 to 15 weight percent of ground fibrous cellulosic material; 5 to 15 weight percent of synthetic chopped fibers; 10 to 20 weight percent of calcium sulfate hemihydrate; and 2 to 12 weight percent of sodium metasilicate.
 14. The lost circulation material composition of claim 1, wherein the lost circulation material forms an unweighted filter cake having a shear strength of up to about 1325 psi at room temperature.
 15. The lost circulation material composition of claim 1, wherein the lost circulation material forms an unweighted filter cake having a shear strength per dtex of the synthetic chopped fibers of up to about 1366 psi/dtex at room temperature.
 16. The lost circulation material composition of claim 1, further comprising a weighting material, wherein the weighting material is selected from the group consisting of barite, salt and calcium carbonate.
 17. The lost circulation material composition of claim 16, wherein the lost circulation material forms a 12.0 lb/gal filter cake having a shear strength of up to about 420 psi at room temperature.
 18. The lost circulation material composition of claim 16, wherein the lost circulation material forms a 12.0 lb/gal filter cake having a shear strength per dtex of the synthetic chopped fibers of up to about 432 psi/dtex at room temperature.
 19. A method of sealing a fracture within a subterranean formation, comprising: preparing a lost circulation material composition comprising: particulate material; fibrous material comprising synthetic chopped fibers selected from the group consisting of polyacrylonitrile fibers and acrylonitrile/methyl acrylate fibers; and non-Portland cement material; mixing the lost circulation material composition with a carrier fluid; and contacting the lost circulation material composition with the subterranean formation such that the lost circulation material composition forms a sealing mass, thereby substantially sealing a fracture within the subterranean formation.
 20. The method of claim 19, wherein the lost circulation material composition seals fractures having a size of at least 200 microns.
 21. The method of claim 19, wherein the lost circulation material composition seals fractures having a size of from 200 microns to 4000 microns.
 22. The method of claim 19, wherein the lost circulation material composition seals fractures having a size of from 500 microns to 2500 microns.
 23. The method of claim 19, wherein the lost circulation material composition seals fractures when it is subjected to a temperature of from 70° F. to 400° F.
 24. The method of claim 19, wherein the lost circulation material composition seals fractures when it is subjected to a temperature of from 150° F. to 250° F.
 25. The method of claim 19, wherein the lost circulation material composition comprises: 45 to 60 weight percent of diatomaceous earth; 5 to 15 weight percent of graphitic material; 5 to 15 weight percent of ground nut shells; 5 to 15 weight percent of ground fibrous cellulosic material; 5 to 15 weight percent of synthetic chopped fibers; 10 to 20 weight percent of calcium sulfate hemihydrate; and 2 to 12 weight percent of sodium metasilicate.
 26. The method of claim 19, wherein the carrier fluid is selected from the group consisting of water, base oil, water based drilling fluid and non-aqueous based drilling fluid.
 27. The method of claim 19, wherein the lost circulation material is added to the carrier fluid in an amount of up to about 70 pounds per barrel.
 28. The method of claim 19, wherein the lost circulation material forms an unweighted filter cake having a shear strength of up to about 1325 psi at room temperature.
 29. The method of claim 19, wherein the lost circulation material forms an unweighted filter cake having a shear strength per dtex of the synthetic chopped fibers of up to about 1366 psi/dtex at room temperature.
 30. The method of claim 19, wherein a weighting material is added to the mixture of the lost circulation material composition and the carrier fluid, the weighting material being selected from the group consisting of barite, salt and calcium carbonate.
 31. The method of claim 30, wherein the lost circulation material forms a 12.0 lb/gal filter cake having a shear strength of up to about 420 psi at room temperature.
 32. The method of claim 30, wherein the lost circulation material forms a 12.0 lb/gal filter cake having a shear strength per dtex of the synthetic chopped fibers of up to about 432 psi/dtex at room temperature.
 33. A system for drilling a subterranean wellbore, comprising: a drill string including a drill bit; and a lost circulation material composition, wherein the lost circulation material composition, comprises: particulate material; fibrous material comprising synthetic chopped fibers selected from the group consisting of polyacrylonitrile fibers and acrylonitrile/methyl acrylate fibers; and non-Portland cement material; wherein the lost circulation material is placed in the subterranean formation through the drill string. 